This invention relates to the roasting of fine grained ores such as cement raw meal, lime, dolomite, gypsum, phosphate rock and the like and more particularly to a method and apparatus for calcining such fine grained materials at a lower temperature or at lower fuel consumption or both than with prior methods and apparatus.
Prior to the present invention, it was well known in the manufacture of cement clinker to process the cement raw meal by utilizing a suspension preheater consisting of a plurality of serially connected cyclone separators followed by a rotary kiln followed by a material cooler. The raw material enters the suspension preheater and is alternately and repeatedly entrained and separated in the stream of hot gases being exhausted from the kiln to thereby preheat the raw meal before it is supplied to the kiln for calcining and clinkering. Utilization of such an apparatus reduces the heat consumption in the cement making process from that of a long dry kiln or a wet process cement manufacturing process.
The next major improvement in the manufacture of cement clinker was the addition of a separate calciner between the suspension preheater and the rotary kiln. The furnace itself can take many different forms, but basically the concept of such furnaces is to maintain in suspension the material to be roasted or calcined while burning fuel in that furnace to maintain the temperature in the furnace sufficiently high to achieve 80-90% calcination of the raw material at a furnace temperature of approximately 900.degree. C. The thus calcined raw material is then supplied to the rotary kiln for the final burning to produce cement clinker.
Higher calcination levels are attainable at furnace temperatures above 900.degree. C.; however, the risk of preheater plugging is increased at elevated temperatures, particularly when using coal as the fuel.
It is known that the degree of calcination is a factor of temperature and the amount of time that a raw material is exposed to that temperature. With apparatus of the type to which the present invention relates in which a suspension type calcining furnace is utilized, temperatures for calcining cement raw meal are on the order of 900.degree. C., but the residence time of the raw material within the calcining furnace and thus the period of time to which the material is exposed to heat at 900.degree. C. is limited to a matter of seconds. Accordingly, if the temperature is increased or the duration of the material exposure to that temperature is increased, the amount of calcination should also be increased. It is known that if the length of time that the material is exposed to the high temperature is increased, then there can be a reduction in the temperature required for a given amount of calcination.
It has been found by the present invention that the degree of roasting or calcination can be increased or in the alternate, the degree of roasting or calcination can be held generally constant at a lower calcining furnace temperature by increasing the number of passes of the material through the calcining furnace. While the residence time of the material within the calcining furnace for any given pass of the material remains the same as with prior apparatus, the increase in the number of passes serves to effectively increase the residence time of the material being roasted within the calcining system.
Because the present system permits lower operating temperatures in the calcining furnace, the gas temperatures throughout the suspension preheater are reduced, and there are lower heat losses with the waste gas and through the vessel walls. The net effect is a reduction in the fuel consumption of the system.
Various efforts have been made to increase the time during which the raw material is exposed to high temperature gases while remaining in suspension. One prior patent utilizes idling chambers which are supplied with a portion of the material which has been heated in a suspension type preheater. The material is subsequently discharged into a rotary kiln for further processing. In that device, however, the kiln exit gas does not flow through the idling chambers to achieve full utilization of the heat from the rotary kiln. In addition, all of the raw material which is discharged from the preheater is not supplied to the idling chamber and, therefore, the idling chamber does not have an opportunity to achieve calcination of the entire raw meal stream.
Another prior art device which includes some recirculation of material being calcined adds fuel to the riser duct carrying kiln exhaust gases to the preheater. Raw material discharged from the preheater is supplied to this riser duct where it is entrained in the hot gases and carried to a gas solids separator. The riser duct serves as both a mixing zone and a combustion zone. The raw material and kiln exhaust gases and combustion gases from the burning of fuel within the riser duct substantially coincide with each other. In this prior art device, separated solids are supplied to either the kiln for final calcination and clinkering or are recycled to the riser duct or portions of the solids are directed to both. This prior device uses a fixed restriction at the inlet of the riser duct and seeks to maintain a constant pressure differential between the inlet and outlet of the riser duct. This pressure differential is used to control the division of material to the kiln or for recirculation through the riser duct combustion zone.
A further refinement of the prior art includes an apparatus wherein material from the suspension type preheater is divided between the suspension calcining furnace and the last stage of the suspension preheater. In this device, however, the full discharge from the last stage of the preheater is supplied directly to the kiln while only a portion of the raw material is supplied to the separate calcining furnace. None of the raw material is recirculated through the separate calcining furnace.
With the present invention, an independent mixing zone for mixing of kiln exit gases and material to be roasted is followed by an independent combustion chamber. The use of independent mixing and combustion zones has the advantage of being able to fully utilize the hot kiln exhaust gases and permits the maintenance of a constant temperature within the calcining furnace itself. With the present invention no attempt is made to maintain a constant pressure in the system as this requires adjustment for varying feed rates and varying gas volumes. The amount of material which is being recirculated is controlled by the level of calcination desired and/or the system operating temperature.
Also according to the present invention it has been discovered that recirculation of the material to be roasted or calcined through the calcining furnace can result in either a higher level of calcination at a given furnace temperature or the same level of calcination can be achieved at a reduced temperature.
Of particular importance to the present invention is the amount of material which is recirculated. In the present invention, a recirculating load of 200%, 300%, 400% and more is contemplated. The recirculating load is defined to mean the quantity of material recirculated through the roasting furnace expressed as a percentage of the quantity of raw material being supplied to the system. The quantity of product being withdrawn from the system is generally equal to the input. By the present invention it has been discovered that with a recirculating load on the order of 400%, a typical cement raw meal can be calcined to a level of 90% at a calcining furnace temperature of 820.degree. C., whereas, with the prior art and no recirculation, the calcining furnace temperature of 970.degree. C. must be maintained to achieve 90% calcination. Similarly at a fixed calcining furnace temperature of 900.degree. C., with 0% recirculation, 80% calcination of cement raw meal is typically achieved, but with 400% recirculating load at 900.degree. C., 98 % calcination is achieved.
Heretofore, in order to achieve nearly 100% calcination of fine lime while the material was in suspension, it was necessary to operate at temperatures near or above the coal ash fusion temperature. The coal ash would then cause a build-up on the equipment causing problems with gas and material flows. When utilizing recirculation as described in this invention, roasting of the ore can take place at a lower temperature. Thus, the roasting of certain materials such as lime can be carried out using coal as a fuel.